The IfcDeepFoundation abstract entity proposed in the IFC 4x2 DRAFT is included within IfcSharedBldgElements, however, this entity and its subtypes are specific to the domain of structural elements.
It is my understanding that both the IfcDeepFoundation entity and it subtype IfcCaissonFoundation should be in IfcStructuralElementsDomain, keeping the same approach currently provided for IfcFooting and IfcPile.
This consideration also applies to their object type (IfcDeepFoundationType and IfcCaissonFoundationType) as well as the IfcCaissonFoundationTypeEnum PredefinedType attribute.
Moreover, IfcPileType is a direct subtype of IfcBuildingElementType in the draft documentation, being necessary to include it as a subtype of IfcDeepFoundationType in the final version